Stretch leveling apparatus and method



Jan. 30, 1962 w. 5. WHEELER ETAL 3,018,817

STRETCH LEVELING APPARATUS AND METHOD Filed March 1, 1960 INVENTORSWALTER (5. WHEELER OLAF B. WENDELL ATTORNEY rts 3,6 l 8,8 l7 PatentedJan. 30, 1&62

3,013,817 E'IRETQH LEVELWG APPARATUS AND METH'GD Walter G. Wheeler, LosAngeles, and Olaf B. Wendell,

El Segundo, Calif, assignors to The Huiford Qorporation, El Segundo,(Ialif, a corporation of California Filed Mar. 1, 1960, Ser. No. 12,1949 Claims. (Cl. 153-35) This invention relates, in general, to metalforming methods and apparatus and, more particularly, to a method andapparatus for straightening, leveling and shaping of flat stock.

In the aircraft industry, as well as other industries, it has been founddesirable to employ for use in certain products sheet metal materialwhich has been subjected to a process of stretch forming, such asstretch leveling, in order to obtain certain desired qualities such asflatness to specified tolerances, freedom from residual stresses,increased or favored mechanical properties, etc. In the past, such sheetmetal material has been stretched leveled by simply gripping the sheetmetal material by opposed jaws and applying a load to the jaws tostretch the material therebetween. However, in a majority of cases,difficulties have been encountered when employing such stretch levelingtechniques since the material has a tendency to shrink or close in uponitself in a lateral (transverse) direction when subjected to a pulllongitudinally. If some prior condition exist in the sheet and does notexist uniformly over the sheet at any transverse section, such aslocalized differences in material strength, thickness, degree offlatness, or amount of induced or residual stress or strain, then theaction of shrinking transversely will tend to create a longitudinal foldor wrinkle in the sheet making it unacceptable for use. For example,thin gage stainless and alloy steel, in a thickness range from .002 inchto .025 inch is vital to the manufacturing of missiles and high Machnumber aircraft. In these products, it is imperative that the sheetstock be very flat, but the nature of the sheet manufacturing processes,such as rolling and heat treating, causes cans and wrinkles in thestock. Steel has a much greater tendency to form cans due to inneraction of elongation and necking forces than softer metals such asaluminum for example. For aircraft and missile applications, eliminationof these irregularities in the material is mandatory.

To obviate difficulties encountered with conventional stretch levelingapparatus, it is desirable to provide an apparatus and method which willprevent the development and formation of the longitudinal folds with aslittle restriction on size and thickness of sheet material as possibleand capable of providing a uniform end product wherein all parts of thesheet material are subjected to the same conditions. In the apparatus ofthe present invention, means are employed for preventing the formationof longitudinal folds which provide the least amount of resistant orloading possible in areas outside of the longitudinal pull itself whenthe sheet material is subjected to the leveling or forming operation.

In accordance with the present invention, the workpiece or sheetmaterial is gripped on opposite ends by two opposing jaws employed toapply longitudinal pull on the material workpiece. The area of theworkpiece between the two opposing jaws is confined by two platensurfaces which confine the workpiece within a specific space, whileallowing freedom of workpiece movement with no applied restraint onlongitudinal or transverse motion. To achieve this, the platen halvesare moved to a position against positive stops such as spacers or shims,or may be mechanically clamped or loaded to provide a gap, spacing orinterval between the opposing platen halves equal to the thickness ofthe workpiece material plus some variable or specific amount ofclearance determined by the workpiece material and the tolerances of theend result desired.

In general, it has been found desirable to employ a gap distance betweenthe platen surfaces which permits engagement of these surfaces with theextreme apexes or high points of the workpiece irregularities which willbend such irregularities to semi-flatten the workpiece. However, it isunderstood that the engagement of the opposing platen surfaces with theheights of the irregularities is not tantamount to compressive loadingforces which engage with substantially the entire surface area of theworkpiece. Thereby, by imparting longitudinal stretching by means of thetwo opposing jaws, the workpiece will be stretched so that the heightsof the irregularities will, in effect, disengage with the platensurfaces having a resultant workpiece which is substantially fiat whencompared to its original condition. An optimum stretch leveled workpiecewhen stretched longitudinally will result in clearance or spacingbetween the surface areas of the workpiece and adjacent platen surfaces.It can be seen, that should a longitudinal fold or wrinkle tend todevelop during the longitudinally stretching of the workpiece, thatportion of the workpiece surface area involved in a fold or wrinkle willcome into immediate engagement with one of the surfaces of the platenand thereby, the fold or wrinkle cannot develop beyond the width of thegap separating the platen surfaces. Since the workpiece or sheet isconfined within the gap between the platen surfaces, the material of theworkpiece developing in the longitudinal fold or wrinkle can developonly to the extent permitted by the gap and, thusly, by controlling thegap between the platen surfaces, the tolerances of the flatness requiredfor the workpiece can be set. No limitation is placed on type orcondition of material, thickness of material, on size of sheet or platennor on the proportions of these elements. The apparatus and method ofthe present invention can be further employed to form parts of variablecross section and to those parts having at least one degree ofcurvature.

Therefore, it is a primary object of the present invention to level orform a sheet metal workpiece employing stretch leveling techniquesincluding means for avoiding the development of longitudinal folds orwrinkles or hats, cans, etc. outside of predetermined tolerances.

Another object of the present invention is to provide a novel apparatusand method for stretch leveling or forming sheet metal materialpermitting normal shrinking transversally of the material duringstretching without developing longitudinal folds, wrinkles, or hatsoutside predetermined tolerances.

Still a further object to the present invention is to provide a novelapparatus and method for stretch leveling sheet metal material whichsubjects the material to the least amount of resistant or loadingpossible from sources outside of the longitudinal pull per se.

Other objects of the invention, and the various advantages andcharacteristics of the present stretch leveling apparatus will beapparent from a consideration of the following detailed description whentaken in connection with the accompanying drawings in which like numbersof reference denote corresponding parts throughout the several views; inwhich,

FIGURE 1 is a perspective drawing of the stretch leveling apparatusshown in accordance with the present invention;

FIGURE 2 is a sectional view of the platen assembly incorporated in theapparatus of FIGURE 1 taken in the directions of arrows 22; and

FIGURE 3 is an enlarged breakaway view in' section showing the platenassembly of FIGURE 2. h

In accordance with the present invention, FIGURE 1 shows a pair ofopposing jaws and 11 suitably carried on the end of arms 12 and 13 whichare activated by a suitable stretch press apparatus. For the purposes ofthe present invention, the conventional stretch press apparatus may takethe form of the commonly referred to swing arm type machine or thestretch forming apparatus may be of the type known as the rotary type.In both types of apparatus, the pair of opposing jaws are powered bysuitable piston and cylinder assemblages to apply longitudinal stretchto sheet metal material or workpiece such as workpiece 14. The oppositeends of workpiece 14 are suitably positioned within the jaws 10 and 11so that sets of teeth 15 associated with each jaw converge upon theworkpiece ends in gripping relationship when the arms of the stretchpress move differentially to apply the stretching force.

During normal elongation of the material by stretching, the material hasa tendency to shrink or close in upon itself in a transverse directioncommencing in the center of the worksheet and progressing out towardsthe opposite ends according to Poissons ratio. The present inventionincludes a material confining means such as a platen assemblyrepresented by the general reference numeral 16 which confinessubstantially the entire workpiece or portion thereof being leveled. Theplaten assembly may be supported 011 the floor below the jaws of thestretching apparatus or a combination of blocks, hydraulic bags, etc.may be employed for supporting the platen assembly about the workpiece.The platen assembly is conveniently arranged so that it may be installedor adjusted about the workpiece after the workpiece has been mounted inthe jaws of the stretching apparatus.

With reference to FIGURE 2, the platen assembly 16 is shown installedabout the workpiece 14 in such a manner that the workpiece is confinedwithin a gap 17 defined by opposing fiat surfaces 20 and 21. The gap isof sufficient distance to permit a slight clearance between the surfacesand the workpiece so that no loading is placed upon the workpiece by theplaten. Approximately .002 inch clearance has been found satisfactory.Consequently, there is no surface friction existing between the platensurfaces and the surface of the workpiece to prevent or inhibitelongation of the material during stretching. The gap may be regulatedby means of a single shim or a plurality of shims or spacers such asshims 22 and 23 which are positioned along the outer extremities of theplaten assembly on the top and bottom thereof.

The platen assembly, in general, comprises a pair of parallel platesrepresented by numerals 24 and 25 which are separated by means of a shimor shims 22 and 23 and which are held together by means of fastenerssuch as bolts 26. However, it is to be understood that the positioningor displacement of the platens relative to each other about theworkpiece may be accomplished by other means than the incorporation ofshims and fasteners. For example, the platens may be mounted on pistoncylinder assemblages such as on a press and may be brought together andstopped to provide a specific clearance between the workpiece and platensurfaces.

In the event the workpiece is provided with irregularities prior tostretching, it is noted that the platens may be brought together so thatthe surfaces of the opposite platens will engage the heights of theirregularities and bend the sheet into a semi-flattened condition priorto the application of longitudinal pull. The contacting of the platensurfaces with such height is not to be considered a compressive force,but only bending movements are developed in the workpiece. These bendingmovements do not relieve the workpiece from residual stresses or strainsand no permanent setting, forming or leveling of the workpiece isachieved.

With reference to FIGURE 3, it is noted that the shims incorporated inthe present invention are provided with a plurality of spaced cut-outs27 which mate with a plurality of holes 28 provided in each of the topand bottom marginal areas of the platens to accommodate passage offasteners 26. Preferably the cut-outs are made to open at one side ofthe shims so that alignment of the platen holes and the shims can beeasily facilitated.

In actual operation, it can be seen that when arms 12 and 13 aredifferentially actuated in the direction of arrows 30 and 31 alongitudinal pull will be developed on workpiece 14 to elongate theworkpiece approximately two per cent to three per cent of elongation. Asthe workpiece is stretched, the center of the workpiece will tend toneck or close in transversely and the material gathered during thisclosing or necking will normally tend to cause the undesirablelongitudinal fold or wrinkle. However, as the fold develops theworkpiece will come into contact with the platen surfaces of the platenassembly 18. This engagement will deter or prevent the fold fromdeveloping any greater than the Width of the gap and the excess metal iscaused to fiow into other areas of the panel instead of adding to thefold. In the event additional hats or cans develop in other areas of theworkpiece while the workpiece is undergoing stretching contact of theworkpiece with the platen surfaces will prevent the development of suchhats or cans beyond tolerable limits as set by the gap width.

In essence, it can be seen through the employment of the apparatus andmethod of the present invention that the flow of metal which normallybuilds up at intermittent areas on the panel during elongation toprovide folds, wrinkles, hats or cans is diverted into other areas ofthe workpiece so that the resultant workpiece is leveled to a degreeheretofore unachievable. For example, it has been found that byemploying the present invention when material such as stainless steel157 or heat treated stain less steel 4340 have been employed, a degreeof flatness 0r levelness can be achieved within .002 when the gapbetween the platen surfaces is .004" plus the thickness of theworkpiece. The material employed in applicants operation has ranged ingages from .008" to .025" including material in an annealed condition.

Inasmuch as the flat plate surfaces defining the gap do not come intocontact with the entire surface of the sheet or workpiece and inasmuchas no compressive load is placed on the workpiece during elongation,conventional stretch forming techniques can be employed including theemployment of conventional stretch forming apparatus. In the presentembodiment of the invention, the platen assembly may support itself onthe floor of the workshop or the platen assembly may be supported onblocks or pads. The shims are employed only to provide the required gapwidth and the fasteners are employed to hold the shims and platen halvesin place. Other types of fasteners may be employed such as clamps forexample, and other stop means than shims may be employed such aselectrical limit stretches which are actuated when the platens reach thespecified gap width.

Various modifications may suggest themselves to those skilled in the artwithout departing from the spirit of our invention, and hence, we do notwish to be restricted to the specific form shown or uses mentionedherein, except to the extent indicated in the appended claims.

We claim:

1. Stretching apparatus for leveling sheet metal material, thecombination comprising, movable means for engaging the opposite materialedges, means in spaced relationship to the material to be leveled forhousing the material and confining the material during leveling, andmeans for moving the movable means to stretch level the panel while soconfined.

2. Stretching apparatus for stretch leveling a sheet metal workpiece,the combination comprising, gripping arrangement for engaging oppositeedges of the workpiece, confining means arranged about the worpiece inspaced relationship thereto for limiting the extent of workpiecedeformity, and means for moving the gripping arrangement in oppositedirection to elongate the workpiece while so confined.

3. Stretching apparatus for leveling an elongated sheet metal workpiece,the combination comprising, gripping means engageable with opposite edgemargins of the workpiece, workpiece confining means supported betweenthe opposite edge margins of the workpiece having a pair of flatsurfaces separated by the workpiece, each flat surface arranged relativeto the workpiece to provide a minute clearance between the surface andthe workpiece to confine the workpiece between the pair of surfaces, andmeans for moving the gripping means in opposite direction to stretch theworkpiece longitudinally while confined between the pair of surfaces.

4. Stretching apparatus for leveling an elongated sheet metal workpiece,the combination comprising, gripping means engageable with opposite edgemargins of the workpiece, a platen assembly disposed between theopposite edge margins of the workpiece including a pair of flat parallelplaten surfaces separated by the workpiece, each platen surface arrangedrelative to the workpiece to provide a minute clearance between thesurface and the workpiece to confine the workpiece between the pair ofsurfaces, means connected to platen assembly to adjust and maintain theplaten surfaces constant relative to each other so that gap therebetweenremains constant during workpiece elongation, and means for moving thegripping means to stretch the workpiece longitudinally while confinedbetween the pair of surfaces.

5. Stretching apparatus for shaping an elongated sheet metal workpiece,the combination comprising, gripping means engageable with opposite edgemargins of the workpiece, a platen assembly for confining the workpiecedisposed between the opposite edge margins; the platen assemblycomprising, a pair of parallel plates arranged on opposite sides of theworkpiece to define a gap therebetween wherein the gap width is minutelygreater than the workpiece gage, means for maintaining the platesstationary relative to each other to provide a constant gap width; andmeans for moving the gripping means difierentially to longitudinallyelongate the workpiece within the gap while confined by the parallelplates of the platen assembly.

6. Stretching apparatus for shaping an elognated sheet metal workpiece,the combination comprising, gripping means engageable with opposite edgemargins of the workpiece, a platen assembly disposed between theopposite edge margins for confining a major portion of the workpiece;the platen assembly comprising, a pair of parallel plates arranged onopposite sides of the workpiece to define a gap therebetween wherein thegap width is minutely greater than the workpiece gage, stop means of apredetermined thickness disposed between the plates along the top andbottom edge margins thereof for maintaining the plates stationary toprovide a constant gap width; and means for moving the gripping meansdifferentially to longitudinally elongate the workpiece within the gapwhile confined by the parallel plates of the platen assembly.

7. The method of stretch leveling sheet metal material, comprising thesteps of, gripping the material on opposite edge margins, confining thematerial to be leveled in an area slightly wider than material thicknessto restrict the amount of material deformation during stretching, andstretching the material longitudinally by differentially pull= ing onthe material opposite edge mar-gins while confining the material withinpredetermined limits.

8. The method of stretch leveling sheet metal material comprising thesteps of, gripping the material on opposite edge margins by grippers,confining the material to be leveled within a given area which is atleast .004 inch greater than the gage of the mate-rial, anddifferentially pulling the grippers to longitudinally elongate thematerial while limiting the confined material deformity or deflectionwithin the given area.

9. The method of stretch forming sheet metal material which comprisesthe steps of, surrounding a portion of the material to be leveled with aform assembly to define a forming area having a width at least .002 inchgreater than the gage of the material so that material deformation ordeflection will be held to a maximum tolerance of .001 inch andstretching the material longitudinally approximately two percent whileso confined, thereby causing material to chow and distributesubstantially evenly throughout the entire sheet of material.

References Cited in the file of this patent UNITED STATES PATENTS933,778 Mueller Sept. 14, 1909 1,264,248 Yoder Apr. 30, 1918 1,863,073Smythe June 14, 1932 2,441,858 Watter May 18, 1948

